Instruments to measure velocity of gases have been the subject of investigation for many years. A common method of measuring velocity of flow involves use of vanes which are rotated by the gas, much like the vanes of a windmill or those of a conventional anemometer. The speed of rotation of the vanes gives a measure of the velocity of the driving medium. Such devices produce an accurate measure of average gas velocity and therefore are highly practical and useful for many applications. Windmill type devices have relatively high inertia and therefore do not react fast enough to give an accurate indication of the velocity of the gas at any given time when the velocity is varying rapidly. Where it is important to know instantaneous velocity, therefore, devices which respond more rapidly (have less inertia) are required.
The attempt to improve overall performance of the windmill type of meters resulted in the hot wire meters, such as the hot wire anemometer for wind, wherein an electric current is passed through a fine conductive wire and the wire is subjected to the flowing gas. Changes in velocity of the gas passing the wire modify the rate of heat transfer between the wire and the gas, thus affecting resistivity of the wire. Thus, changes in velocity of the surrounding fluid result in changes in temperature and, consequently, resistance of the wire which provide an indication of velocity. The hot wire meter or instrument does, in fact, represent a significant improvement over the earlier windmill type but still has disadvantages, one of which is a certain amount of inertia. A discussion of hot wire anemometry and a bibliography on the subject is found in Mark V. Morkovin, "Fluctuations and Hot-Wire Anemometry in Compressible Flows" (North Atlantic Treaty Organization, Advisory Group for Aeronautical Research and Development, AGARDograph No. 24, November 1956).
Another refinement is represented by the corona discharge anemometer, wherein a high voltage is applied to a sharp point exposed to the atmosphere. The discharge or corona current from the point depends upon the wind velocity at the point. The current is measured and used as an indication of wind speed.
The present invention is an improvement over the now known corona anemometers. It is known that when the potential applied to the point of the corona discharge anemometer is negative, the nature of the discharge is such that it is pulsed, and for a given set of conditions, the pulses occur with great regularity. Further, each pulse carries the same quantity of charge, and the pulse repetition frequency (PRF) rate is determined by the potential or field distribution in the vicinity of the point. The pulses are often referred to as Trichel pulses, after G. W. Trichel, an early investigator of the phenomenon. For a discussion of the pulsed nature of the discharge and the effect of wind speed on discharge current, see J. Alan Chalmers, Atmospheric Electricity (2nd ed., Pergamon Press, 1967), pp 241, 251-252.
If there is no flowing gas (e.g., wind) in the region of the discharge point, only the electric field removes the space charge generated by a Trichel pulse, thus permitting a succeeding pulse to occur. A flowing gas, however, also removes space charge. The present invention relies upon the fact that the variations in pulse rate and, consequently, in current are caused by removal of space charge within a very small region close to the point, which in turn results in very rapid response to fluctuations in wind or gas velocity. The improvement further relies upon the realization that the PRF changes in a manner dependent upon the speed of gas flow (e.g., wind speed) and that changes in discharge current are in fact the result of variations in the PRF, and therefore the PRF is a more fundamental measurement of variation of velocity than is current variation.
The corona current as measured by a conventional microammeter is the total charge passing divided by the period involved; the response is such that only an effective average over a long time period is obtained. On the other hand, the almost instantaneous current is the charge (constant) per Trichel pulse divided by the time separation between pulses; this is exactly equivalent to the product of charge per pulse and PRF. Thus, the PRF is highly responsive to rapid velocity changes.
It is often of particular importance to detect the velocity variations associated with changes from laminar to turbulent flow and then to assess the degree of turbulence. The rapid response afforded by the present invention provides a way to detect such velocity variations.
The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing.